This invention relates to a method of modifying and controlling the performance results of a Fischer-Tropsch Synthesis process. Particularly, this invention relates to a modified method of predicting, controlling and thus improving the product selectivity of the High Temperature Fischer-Tropsch synthesis process, and more specifically, the selectivity of the olefinic fraction of the product spectrum.
Fischer-Tropsch processes are known to produce gaseous and liquid hydrocarbons as well as oxygenates containing, amongst others, paraffins, olefins, alcohols and aromatics, with a variety of carbon chain length ranges and isomers, which, in general, follow the well-known Anderson-Schulz-Flory distribution. Much emphasis has been placed on the modification endeavors, more particularly to improve, as well as maximize the selectivity of the unsaturated hydrocarbons, especially olefins in the C2-C4 range. whilst maintaining high activity and stability under the normal Fischer-Tropsch synthesis conditions with an iron catalyst. Equation 1 is a general presentation of the Fischer-Tropsch reaction:
CO+(1+x)H2xcfx84CH2x+H2Oxe2x80x83xe2x80x83(1)
The reaction can be carried out in fixed, fluidized or slurry bed reactors. The production of olefins and petrol range products is most favoured by synthesis carried out in a two-phase fluidized bed reactor operating at xcx9c350xc2x0 C. and 20 bar or higher pressures and utilizing a fused promoted iron catalyst. The fused iron catalyst is typically promoted with alkali chemical and structural promoters. As a result of the high temperatures which are used in these reactors, they are known as High Temperature Fischer-Tropsch (HTFT) reactors, thus distinguishing them from fixed bed and slurry bed reactors (Low Temperature Fischer-Tropschxe2x80x94LTFT), which operate at temperatures which are about 100-150xc2x0 C. lower than the said HTFT process.
The HTFT process also utilizes a technique which facilitates online removal of spent catalyst and online addition of fresh catalyst to maintain catalyst activity and selectivity profiles at levels which are as favourable as possible. This technique is aimed at achieving an equilibrium performance and also inhibiting the occurrence of undesirable and negative sudden changes in synthesis performance; thus providing a means through which the product spectrum demands, as dictated by the market forces and downstream requirements, can be met.
The Fischer-Tropsch process is known to be directly influenced by process conditions, for example, feed composition, feedrate, conversion, reaction pressure and temperature. In addition, and particularly for the HTFT process, the chemical composition of the catalyst used in the synthesis process has been shown to have a direct influence on the said product spectrum. Thus the concentration level of the chemical components of the Synthol catalyst matrix, such as sodium, potassium, alumina, silica and the like, has been shown to have a direct correlation with yields and the selectivities of the olefins, paraffins, acids and the oils produced in the process. A number of reports have been published which claim that potassium increases the alkene content of the hydrocarbon products, increases the rate of the water-gas shift reaction and suppresses methane formation.
The applicant has surprisingly found a method of modifying and controlling, and thus improving, the selectivity profile in favour of the desired Fischer-Tropsch synthesis products. Particularly, the applicant has found a unique method of manipulating, and thus improving, the selectivity profile of the lower olefins produced by means of a High Temperature Fischer-Tropsch process.
The method is characterized in that predetermined amounts of promoter-carrying compound either dissolved in solution or in a powdered form are directly injected into the reactor medium, typically into the reactor feedstream. A typical chemical promoter for the HTFT process is potassium. The applicant has found that by adding or doping the reaction medium with the promoter-containing compound during the synthesis process, the promoter being potassium, the selectivity profile of the olefins and the paraffins in the product stream is significantly changed, with more olefins being formed whilst the level of paraffins is reduced.
Analysis of the iron catalyst sample has surprisingly shown that within the catalyst matrix, potassium is the most mobile component in the solid solution. The Scanning Electron Microscope (SEM), Energy Dispersing X-ray (EDX) and Secondary Iron Mass Spectrometry (SIMS) techniques have convincingly shown that, with time online, a fraction of the potassium promoter continuously migrates away from the iron metal nuclei, to an extent that it is eventually lost altogether from the matrix and is ultimately captured, for example, in the carbon mass deposit that is formed around the catalyst particle during the synthesis process. The applicant has found that the potassium promoter becomes diluted by migration into the mass of the continuously forming elemental carbon around the catalyst particle with time online.
Furthermore, the analysis has surprisingly revealed that the catalyst particles do not contain a homogeneous concentration of the potassium promoter i.e. the amount of potassium contained in the catalyst particles progressively follows a Gaussian trend. Surprisingly, this applies to catalyst particles of the same size. The effect hereof is that some particles have very low levels of K2O. This appears to be an inherent problem which originates from the procedure that is used in the preparation of the catalyst (fusion process).
The applicant has found that when physically adding potassium into a HTFT reactor during the synthesis process, the added potassium replenishes the xe2x80x98lostxe2x80x99 potassium in the catalyst matrix, and in the process the product spectrum becomes more olefinic. The potassium that is added online is in the form of a compound dissolved in solution or in a pulverized state, the compound selected from potassium carbonate and potassium silicate. This added potassium distributes itself homogeneously through all the catalyst particles inside the reactor, boosting those particles which initially contain very little K2O.
The applicant has further found that an expression which combines the concentrations of the previously mentioned catalyst components, known as the selectivity factor, can be successfully used in correlating the selectivities and the yields of the olefins, paraffins, and thus the olefin/paraffin ratios. Previously, such correlations could not be established, so that it was virtually impossible to predict the yield and the selectivity profiles of the Synthol product spectrum. The applicant has also shown that, to a reasonable degree of accuracy, the selectivity levels of the olefins, as compared to the paraffins in the product stream, may be sufficiently predicted based on the amount of potassium added in the solution prepared for injection.
Accordingly, according to a first embodiment of the invention there is provided a method for controlling a selectivity profile of products of a Fischer-Tropsch synthesis process, the method including the step of introducing into a Fisher-Tropsch reaction medium, during the synthesis process, a catalyst promoter or substance, composition or salt containing the catalyst promoter.
The Fischer-Tropsch process is preferably a High Temperature Fischer-Tropsch process, and the catalyst promoter may be introduced into a fluidized bed Fischer-Tropsch reactor feedstream or at any other suitable location.
The catalyst promoter may be a promoter for an iron catalyst. The catalyst promoter may be a Group I element, more particularly the Group I element may be potassium or a salt or compound thereof.
An alkali promoter-containing compound may be used to introduce the catalyst promoter into the feedstream, the promoter-containing compound including an oxide or salts thereof. Typically, the promoter-containing compound is a potassium oxide or a potassium halide. More particularly, the promoter-containing compound may be potassium carbonate, potassium silicate or potassium bromide, preferably potassium carbonate. The promoter-containing compound may be in the form of a solution or a powder.
The selectivity profile of olefins, preferably olefins in the C2-C4 range, may be increased by the addition of the catalyst promoter to the reaction medium during the synthesis process The selectivity factor may relate to the catalyst composition, and conversely the catalyst composition may be determined according to the required selectivity factor. More particularly, the selectivity factor may relate to the potassium oxide, alumina and silica concentrations within the iron catalyst, and even more particularly does not relate to the sodium oxide composition of the catalyst.
The selectivity factor (SF) may be expressed according to the following equation:                     SF        =                              (                                          K                2                            ⁢              O                        )                                (                                                            Al                  2                                ⁢                                  O                  3                                            +                              SiO                2                                      )                                              (        3        )            
It will, however, be apparent to a person skilled in the art that this is not the only equation which may be used to determine the selectivity factor.
The quantity of additional catalyst promoter required to achieve the desired selectivity factor may be calculated according to the following equation if K2CO3 is used as the promoter-carrying compound:
p=[ReqComprx(SiO2+Al2O3)xe2x88x92K2O]/0.68xe2x80x83xe2x80x83(4)
where p is the amount of additional potassium promoter per 100 g Fe;
SiO2, Al2O3 and K2O refer to the composition of the spent catalyst to be modified; and
ReqCompr is used to indicate the SF which corresponds to a catalyst with the desired olefin selectivity.
If a compound other than K2CO3 is used (e.g. K2SiO3), then the equation should be modified in accordance with the molecular weight of the specific compound.
According to a second embodiment of the invention there is provided a Fischer-Tropsch catalyst system having a desired olefin selectivity factor, the catalyst system including a quantity of catalyst promoter related to the catalyst composition and the selectivity factor.
The catalyst and a catalyst promoter may be substantially as described above. The promoter may be selected from a group including potassium oxide, alumina, silica and sodium oxide.
The selectivity factor may be related to the iron catalyst promoter""s potassium oxide, alumina and silica concentrations, and preferably not necessarily to the sodium oxide concentration. The selectivity factor may be determined substantially as described above, as may the quantity of catalyst promoter be determined.
According to yet a further embodiment of the invention there is provided a method of maintaining a selectivity profile of products of a Fischer-Tropsch synthesis process within a preselected range the method including the step of introducing into a Fisher-Tropsch reaction medium, during the synthesis process, a catalyst promoter or substance, composition or salt containing the catalyst promoter.
The method may be the same or substantially similar to the method of modifying and controlling the selectivity profile described above.
It is on the basis of the favourable promotional effects of potassium that an optimum amount thereof is maintained consistently within the catalyst inventory during the synthesis reaction. Under HTFT conditions the iron catalyst particles are known to continuously show the deposition of elemental carbon occurring around them. When carbon is deposited on the iron catalyst is the particles swell arid also disintegrate. In a fluidized catalyst bed in particular, the fines which are produced as a result of catalyst disintegration have a high carbon content and hence have a low particle density. The fine, low density particles are also preferentially lost via cyclones during the synthesis, thereby lowering the available alkali levels even further.
At this stage the potassium is diluted (in terms of K2O unit volume) inside the catalyst particles, and the amount of potassium that is in contact with iron is lowered. In practice, as the synthesis reaction progresses, the amount of potassium promoter within the catalyst particles gradually diminishes due to the high mobility rate of this particular promoter within the solid solution. As the level of the active potassium within the catalyst matrix progressively decreases, the catalyst itself, complementarily, becomes more selective towards a paraffinic hydrocarbon product. As a result, a desired Fischer-Tropsch product selectivity profile wherein the olefinic fraction is dominant cannot be maintained indefinitely if the diminishing potassium content continuously alters the catalyst effective composition.
The applicant has found a method of continuously maintaining the product yield and selectivity of the HTFT process using an iron-based catalyst, wherein the olefinic product content is dominant. The method of the present invention includes the physical injection of a potassium containing compound, for example potassium carbonate and potassium silicate, into a Fischer-Tropsch fluidized bed reactor feedstream or into the reaction medium at any other suitable location, whilst the process operation is in progress. The injection of the potassium-containing compound is capable of immediately, or substantially immediately, restoring the otherwise declining olefin selectivity levels to the original maximum levels.
The present invention also provides a method for determining, to within a reasonable degree of accuracy, the selectivity levels of the olefins and the paraffins within the hydrocarbon product stream. Previously, it was believed that it was feasible to correlate, with a variable degree of precision, the iron-based Synthol catalyst composition, also known as the catalyst type, with the olefin (and paraffin) product selectivity. The following expression, which is known as the selectivity factor (SF), combines the concentrations of the promoters and the support components of the iron catalyst:                     SF        =                              (                                                            K                  2                                ⁢                O                            +                                                Na                  2                                ⁢                O                                      )                                (                                                            Al                  2                                ⁢                                  O                  3                                            +                              SiO                2                                      )                                              (        2        )            
wherein each oxide is expressed per 100 g Fe. This factor has traditionally been used, with limited success, as a measurement entity to test whether the hydrocarbon product selectivity profile correlated directly with the catalyst composition. However, it has now been established that the influence of potassium on the selectivity profile is much more pronounced than that of sodium, such that the sodium component concentration, once present in similar amounts to that of potassium, can be excluded from the selectivity factor expression, thereby improving the accuracy of the selectivity factor under these conditions. The new selectivity factor is now expressed as follows:                     SF        =                                            K              2                        ⁢            O                                (                                                            Al                  2                                ⁢                                  O                  3                                            +                              SiO                2                                      )                                              (        3        )            
Analysis of the synthesis results shows that there is indeed a direct correlation between the modified selectivity factor and the olefin selectivity as well as the olefin: paraffin selectivity ratios. Expressed differently, if the concentrations of the alumina and silica support components are constant, there exists a direct correlation between the potassium promoter concentration in the catalyst matrix and the olefin selectivity and the olefin: paraffin selectivity ratio. Therefore, a decrease in the amount of active potassium correlates to an increase in selectivity of the paraffinic hydrocarbons, and consequently a decrease in the olefin: paraffin ratio.
By physically injecting an alkali compound containing potassium, such as potassium carbonate or potassium silicate, into a Fischer-Tropsch process operating with a catalyst in a fluidized bed mode, the diminishing catalyst is potassium promoter content is suddenly replenished, and in line with phenomena discussed above, the selectivity profile of the synthesis process is such that the olefin selectivity, and thus also the olefin: paraffin selectivity ratios correspondingly increase. The alkali promoter injection method as herein described is carried out online, and the compound is added either in a solution or a powdered form at a selected injection point to allow mixing with the total feed gas prior to entrance into the reactor. The alkali promoter injection method also increases the synthesis gas conversion capacity. Characteristically, the acids in the reaction water as well as in the unstabilized light oil (ULO) fraction are also increased, whilst the alcohols and the carbonyls remain substantially unchanged.
Furthermore, the process of the present invention, through the results obtained, provides an additional or alternative means of countering negative or poisoning effects of the coal-derived synthesis gas side components, such as sulphur, specifically as regards to the question of catalyst selectivity. The iron catalyst is characteristically sensitive towards the increase in the levels of sulphur as sulphur is preferentially adsorbed onto alkali-rich sites on the catalyst surface thereby rendering them ineffective. Thus if high amounts of sulphur are present in the synthesis gas the catalyst activity and selectivity are severely affected. The present invention, wherein fresh alkali is injected into the catalyst bed, counteracts the negative effects of sulphur poisoning by re-instating the presence of fresh alkali-rich and active sites.
The invention will now be illustrated by means of the following non-limiting examples and with reference to the accompanying figures.